Radar measurements of the total cislunar electron content

During three nights and two mornings at the end of December 1964, two lunar radars were used to make the first measurements of the total integrated electron density between the earth and the moon. The measurements were accomplished by determining the difference in time delay between 25- and 50-Mc/s moon-reflected signals to an accuracy of 10 μsec. The experiment was made possible by the use of FM radar transmissions with time-bandwidth products of 105. Subtraction of ionospheric electron contents from the totals determined from the differential group delay measurements suggests that there are two distinct regions beyond the ionosphere. Within the solar wind wake of the earth, in the quadrant opposite the sun, the average cislunar electron density beyond the ionosphere is approximately (300±50) cm−3. In the general direction of the sun, where most of the radar path is beyond the shock wave boundary, the average density is much lower, (100±15070)cm−3. Ionospheric and magnetospheric contents and experimental uncertainty are discussed. Though only a few measurements have been made, the initial results clearly show a marked effect, which is interpreted here as an average electron density inside the shock front, in a magnetospheric wake extending at least as far as the lunar orbit, which exceeds the solar wind density by about 200 cm−3.